There is no official source saying so however we can speculate.
While the Emperor never really shared much about his true goals we do know one thing, he wanted to destroy the chaos gods but he first seemingly needed to conquer most of the milky-way galaxy in order to do so.
It is my personal belief that some(maybe all) the primarchs were created in some extend to fight the chaos gods, Lion el Johnson for example could ignite his weapons with white psychic fire when he fought the deamons that invaded his ship the invincible reason.
It is also heavily hinted that all primarchs are themselves a fusion between warp and reality, for when Corvus Corax entered the warp he could transform himself into a flock of ravens. Coupling with the fact that every primarch seemingly is the equal or greater than the greater deamons of chaos they paint a eerily comparison that they themselves are greater deamons of the Emperor of some sort.
This is pure speculation on my part but i believe the Emperor wanted to first take over the galaxy and then tell his sons about chaos and how they were going to fight it. But as we all know he never got the chance to do so because someone screwed everything up.
So in conclusion i think he avoided preparing his sons to fight the warp until he himself was ready to take the fight to the chaos gods.(which ultimately allowed their corruption because they did not even know the warp housed sentient beings )